Sub-Saharan Africa has the world's highest rates of maternal and perinatal mortality and accounts for 2/3 of new HIV infections and 1/4 of preterm births. Antenatal (prenatal) care is the entry point into the health system for many women and offers a unique opportunity to provide life-saving monitoring. However, provider shortages, low quality of care and failure to attend all recommended visits mean that the potential benefits of antenatal care are not realized. There is an urgent need to test novel interventions to reduce health risks for mother and child. Group antenatal care is a transformative model of care that provides a positive pregnancy experience, uses provider time efficiently, and improves perinatal and HIV-related outcomes. Women in group antenatal care have 2-hour visits with the same provider in a group of 8-12 women at a similar stage of pregnancy. Women conduct self-assessments, briefly consult the midwife, and meet for 80-90 minutes of interactive health promotion enlivened by games and role-plays. Women form relationships with midwives and each other. In a US randomized clinical trial (RCT), group care improved prematurity rates, antenatal care attendance, satisfaction with care, breastfeeding practices, safer sex behaviors, and uptake of family planning. Our randomized pilot in Malawi and Tanzania had promising outcomes. More women in group care than in usual care completed ?4 antenatal visits (94% vs 58%). Their partners were more likely to be tested for HIV during pregnancy (51% vs. 27%). We established that group antenatal care can be offered in a rigorous RCT with high fidelity despite provider shortages. The next step is an adequately powered effectiveness trial. Malawi is an especially appropriate site because it has the world's highest prematurity rate (18%) and high HIV prevalence (10% nationally, 16% at the study site). We use a hybrid design to simultaneously conduct an effectiveness RCT with individual-level randomization and examine implementation processes at 6 clinics in Blantyre District, Malawi. Aim 1 is to evaluate the effectiveness of group antenatal care through 6 months postpartum. We hypothesize that compared to usual care, women in group care and their infants will have less morbidity and mortality and more positive HIV prevention outcomes. We test Aim 1 hypotheses using multi- level hierarchical models using data from repeated surveys and health records. Aim 2 is to identify clinic-level degree of implementation success and contextual factors associated with success for each clinic and across clinics. Analyses use within and across-case matrices. This high-impact study addresses three global health priorities, maternal and infant mortality and HIV prevention, that affect all women of childbearing age in Malawi. The Ministry of Health strongly supports this project; results will help them decide whether to scale-up this innovative model of group care. Negative results will avoid spending on ineffective care. Positive results will provide evidence needed to adopt group antenatal care nationally and in other low-resource countries.